This invention relates to a dry developer mix for developing charge patterns created by means of electrophotographic imaging techniques and, more particularly, the invention relates to a developer mix of the character described, having improved fatigue resistance and imaging properties.
In the practice of electrophotographic processes, an electrostatic charge is produced on the surface of a photoconductive member. The construction of these photoconductive members is well known, and one example of such photoconductive members involves the use of inorganic photoconductive metal ion containing crystalline materials, which are dispersed in a resin binder. The compounds which may be used in the instant invention and which fall within this definition of photoconductive insulating compounds are zinc oxide, zinc sulfide, zinc-cadmium sulfide, zinc-magnesium oxide, cadmium selenide, zinc silicate, calcium-strontium sulfide and zinc titanite.
In addition to inorganic photoconductive compounds, the photoconductive member may be formed by using organic photoconductive materials, which in themselves are film forming and may not require an insulating resin binder. Such materials as polyvinylcarbazole, polyacryloylcarbazole, polyacryloylbenzocarbazole, vinyl copolymers containing vinylcarbazole units.
It will be understood that the actual formation of the photoconductive member comprises no part of the instant invention and, therefore, will not be discussed in greater detail.
The charge pattern or latent image is created on the photoconductive member by first exposing the charged surface to a pattern of light and shadow wherein the light rays affect the discharge of the exposed portions of the surface while the portions of the photoconductive layer corresponding to the shadow portions retained the charge. The resulting latent image is then rendered visible by applying thereto toner particles, which are attracted to the charge portions of the surface or the image portions.
The mechanisms of the development of the latent electrostatic images, as a technique, are well-known. Depending on the type of photoconductive member it will be imparted a blanket electrostatic charge which is either positive or negative. It will then be exposed to a pattern of light and shadow and developed with a developer mix in which the charge orientation of the toner particles, as they are applied from the developer mix, will appropriately be attracted to the image portions of the electrostatic latent image. Such electroscopic powders may be utilized to develop positively charged images as well as being attracted to those portions which are light exposed by virtue of the charge orientation of the toner particle.
Having rendered the electrostatic charge pattern visible on the photoconductive member the final copy may be produced in one of two ways. In the circumstance that the photoconductive member itself is to be the final copy then the powder image is fixed directly on the photoconductive surface by heating the member causing the thermoplastic resin particles to coalesce, or, it may be fixed onto the photoconductive member by pressure, or it may be fixed by high intensity flash radiations such as that produced by a xenon radiation source, or by exposing the thermoplastic resin to solvent vapors which will cause the material to partially dissolve and flow together. Other techniques call for the transfer of the powder image from the photoconductive layer to a plain paper receiving sheet or other suitable receiving member and the transferred powder image is fixed thereon by any one of the previously mentioned techniques.
The two principal ingredients of a dry developer mix are the toner particles and the carrier particles. The toner and carrier particles coact with one another so that there is produced a tribo-electric charge causing the carrier and toner particles to be attracted to one another. It is by this technique, whereby tribo-electric charge is developed on the toner particle, that causes it to be attracted to the charge pattern on the photoconductive surface by a force which is greater than the triboelectric force which holds it to the carrier particle.
It is to be understood that the benefits of the instant invention are to be realized by applying it to developer mixes which can be used in the environment of a magnetic brush or a cascade system or any other system wherein the carrier and the toner particles are dissimilar materials permitting the development of the tribo-electric charge. While the instant invention is described in the environment of a magnetic brush system, which depends on the use of magnetically attractable carrier particles, it is intended that it not be limited thereto and could be used to equal advantage where the carrier particles are glass beads or any other material.
The conventional electrostatic systems in use today generally keep the developer mix in a closed system. This means that the materials are recirculated from a supply source to the developing station and the unused materials are returned to the supply system and thereby recycled. The toner particles are consumed from the developer mix upon being attracted to the image portions of the electrostatic copy. Consequently, the concentration of toner in the developer mix becomes depleted, that is, the concentration of toner to carrier during the course of the reproduction or duplicating cycle changes. It therefore becomes necessary to replenish the amount of toner in the developer mix in order that the proper level of concentration be maintained in order to optimize the performance of the developer mix.
Understandably, the replenishment of the carrier materials occurs much less frequently since the carrier itself is not consumed in the process. However, there is some loss of the carrier and it, too, requires replenishment.
One of the serious deficiencies of the dry electrostatic developer mixes is the tendency for the mix to "fatigue." A developer mix will be recognized as becoming fatigued when it fails to perform in the following described manner. There occurs a significant fall off in the density of the image (degree of blackness) which is produced; that is, a very dense black image tends to become increasingly gray. The contrast begins to degrade whereby the background or non-image area begins to accept toner particles producing a black-on-gray background or a dark gray on a light gray background instead of black-on-white. Attempts on the part of the operator to correct the copy quality by replenishing the supply of toner will have no effect because the system fails to respond to increases in concentration, and at some point it will tend to increase the background. Thus, a fatigued developer mix appears to have lost its all-important properties, namely, to seek out the charge or imaged portions of the photo-conductive member as opposed to the non-image or background of the sheet at a reasonable concentration of toner in the system.
U.S. Pat. No. 3,320,169 issued May 16, 1967 to the assignee of the present application recognizes the problem of developer mix fatigue or drop-off as a problem of longevity and discloses that by the addition of certain fatty acids, fatty acid salts and mixtures thereof, the life of a developer mix is increased from the production of 2000 to 6000 copies and even up to 10,000 copies. At the time the development reported in U.S. Pat. No. 3,320,169 was made, the output of the equipment to which the invention was directed was required to make reproductions at a rate of about 8 to 15 per minute.
The advent of higher output equipment, for example those which must produce in the range of from 40 to 60 copies per minute, has imposed still greater stress and made still greater demands on the formulation of developer mixes and has greatly intensified the problem of mix fatigue than had been experienced by the equipment up to that time.
It will be appreciated that these systems are recycled so that the movement of the developer mix through the developer system, whether it be cascade or magnetic brush system, is tumbled at a rate which is commensurate with the speed of movement of the copy sheet through the equipment. Accordingly, in a machine which turns out 60 copies per minute, the developer will be moving 3 to 4 times as fast as a machine which is producing 8 to 15 copies a minute. An increase of 400 and 500% in the rate of movement of this developer has subjected the developer mixes to much greater stresses and forces which cause the fatigue of the materials. It is theorized that at these great speeds the toner particles experience further comminution, which is one reason why they tend to lose their charge orientation. The carrier itself tends to have permanently bonded onto its surface, due to the pressures involved, the thermoplastic materials so that the proper degree of tribo-electric force to be developed suffers greatly since the carrier is now coated with the same material as the toner particle itself.
The use of the fatty acids and the metal salts of the fatty acids has served well the cause of controlling fatigue but in the environment of such high production machines has been found to be depleted more rapidly from the system. This requires the use of high concentrations of the metal soaps which, it has been found, could interfere with the orientation of the toner, and in addition, requires higher temperature to fix the powder to the base support.
It is therefore apparent that there is a need for dry developer mixes having improved fatigue resistance, especially in the environment of the high output duplicating and copy machines. The present invention is addressed to filling this need.